1. Field of the Invention
The invention relates to methods for producing, purifying and recovering orthophosphoric acid. More particularly, the invention relates to improved methods for producing stabilized wet process phosphoric acid having significantly reduced levels of magnesium and aluminum.
2. Description of the Prior Art
Phosphoric acid is a versatile chemical finding its way into diverse industries. The fertilizer industry neutralizes phosphoric acid with ammonia to provide a rich source of nitrogen and phosphorous readily assimilable by food crops. Nitrogen and phosphorous have many nutritional values, the most notable perhaps being as structural components of nucleic acids. Accordingly, providing an adequate supply of nitrogen and phosphorous to food crops insures cell multiplication and thus, growth.
It is essential that the fertilizer not have any appreciable amounts of precipitatable metal contaminants which when precipitated present sludge problems. When taken up in food plants the presence of the metal contaminants may be undesirable. Raw phosphoric acid contains metal contaminants such as magnesium and aluminum in substantial amounts.
Most phosphoric acid produced in the United States is produced by the wet process. In this process finely ground phosphate rock is slurried with sulfuric acid and recycled dilute phosphoric acid. The products produced by the chemical reaction between the phosphate rock and sulfuric acid are orthophosphoric acid (commonly known in the industry as phosphoric acid, or more particularly, as wet process phosphoric acid), gypsum, and numerous other suspended and dissolved impurities. The slurry produced is filtered to remove solid impurities, of which gypsum is the major constituent. The resulting filtrate usually contains between about 25 and 35% by weight P.sub.2 O.sub.5, and between about 1 and 8% by weight suspended solids or impurities that are not removed by filtering. This dilute wet process acid is usually concentrated in multiple stage evaporators to about 52-54% P.sub.2 O.sub.5 prior to storage and eventual shipment.
The problems encountered in the industry with the impurities produced by the phosphate rock-sulfuric acid chemical reaction are well-known. One major problem encountered is that the impurities not removed by filtration settle out during processing, storage and shipment to form a hard voluminous sludge. This sludge formation is known in the industry as post precipitation. The sludge is difficult and costly to remove and, additionally, represents substantial losses of P.sub.2 O.sub.5 values. One of the major constituents of the sludge is the complex salt (Al, Fe).sub.3 KH.sub.14 (PO.sub.4).sub.8.4H.sub.2 O. Concentrating the acid to more desirable P.sub.2 O.sub.5 levels compounds the post-precipitation problem. When the wet process phosphoric acid is neutralized with ammonia, magnesium-containing solids precipitate out of solution clogging the transfer and distribution equipment, reducing P.sub.2 O.sub.5 values and hindering the removal of other types of impurities.
Magnesium and aluminum are two particularly unpleasant impurities present in wet process phosphoric acid. Most magnesium and aluminum present in phosphate rock is dissolved either during the reaction of the phosphate rock with sulfuric acid, or during the filter wash cycle. Once in the acid, the two impurities have proven very troublesome and difficult to remove. Both impurities have unfavorable effects on acid viscosity. The detrimental effect of magnesium on deposit formation during ammonia neutralization steps was previously discussed.
Offsetting these problems is the fact that phosphoric acid is an excellent phosphate donator and the wet process is an economically feasible method for obtaining phosphoric acid. Accordingly, a substantial amount of technology has been generated to solve the numerous problems associated with cleaning up wet process phosphoric acid.
A number of processes have been employed to reduce post precipitation in wet process phosphoric acid. Settling is often employed to reduce the sludge content of the acid prior to shipment. Settling, however, does not resolve the post precipitate problem easily or economically because of the lengthy time required and, also, because acids clarified solely by settling, may still exhibit post precipitation tendencies later on. Purification schemes such as chemical precipitation, solvent extraction and chromatographic ionic exchange methods are not amenable to large scale industrial production.
Stabilization of wet process phosphoric acid is one of the strategies used to clean up wet process phosphoric acid. Examples of stabilization strategies can be found in the U. S. patents issued to Richard Hill. They are U.S. Pat. Nos. 4,110,422; 4,164,550; 4,248,846; 4,279,877; 4,293,311; 4,305,915 and 4,364,912.
U.S. Pat. Nos. 4,110,422 and 4,164,550 to Hill describe a process in which a stabilized wet process phosphoric acid is produced by addition of an aluminum silicate material such as perlite to clarify dilute phosphoric acid, concentrating the acid, transferring it to a crystallization zone where additional clarification occurs, and then further concentrating the acid. This process is not directed toward reducing the magnesium and aluminum content of the acid and may leave high concentrations of the two metals in the acid product.
U.S. Pat. No. 4,248,846 further incorporates a recycle stream from the crystallizer underflow to the acid train and provides for the addition of sulfuric acid to the evaporators when processing rock high in iron and aluminum and for the cooling of one or more streams of the process. This process produces acids which may also be high in aluminum and magnesium.
U.S. Pat. No. 4,279,877 provides a process for high-iron feed acid in which some of the iron is present in the ferrous form and teaches the use of an oxidant such as hydrogen peroxide to oxidize all ferrous iron to the ferric state. The treatment reduces post-precipitation of the final product acid; but, the final product may still be high in magnesium and aluminum.
U.S. Pat. No. 4,293,311 also modifies the process of Hill U.S. Pat. No. 4,110,422 by incorporation of a crystallizer underflow recycle stream to the aluminum silicate addition vessel. Aluminum silicate is still required and this process produces acids which may still be high in magnesium and aluminum.
U.S. Pat. No. 4,305,915 teaches a process similar to those described above and is directed to reduction of post-precipitation tendencies. This particular process requires the addition of perlite or chemicals such as aluminum silicate. Acid products high in magnesium and aluminum may still be produced.
U.S. Pat. No. 4,364,912 teaches the production of a stabilized acid without the use of aluminum silicate by prescribing a Fe.sub.2 O.sub.3 -to-P.sub.2 O.sub.5 weight ratio lower than 0.03 in the feed acid with all the other steps of the process of the Hill U.S. Pat. No. 4,110,422 unchanged. The final product may still have high concentrations of magnesium and aluminum.
More selective chemical purification schemes are known. U.S. Pat. No. 2,954,287 to Carothers et al teaches the purification of wet process acid by the addition of an alkali salt to the sulfuric acid used to attack the phosphate rock. Impurities are advantageously precipitated, it is taught. However, the process of this patent is directed to removing iron, aluminum and fluorine and not magnesium.
U.S. Pat. No. 3,408,162 to Satterwhite et al teaches the prevention of post precipitations by adding a lignosulfonate to the wet process acid. The process, it is taught, keeps impurities solubilized which would otherwise spontaneously precipitate out of solution. The end product may be highly impure and may contain high amounts of magnesium and aluminum.
U.S. Pat. No. 3,512,927 to Betts is directed to recovering fluosilicates from the wet process phosphoric acid and converting the fluorine in the fluosilicates to a soluble fluoride form and recovering another fluorine fraction from the wet process acid in the form of a soluble compound of aluminum and fluorine. The two forms of fluorine are then united in a solution containing sodium, potassium and ammonium which results in the precipitation of aluminum and fluorine from the final product. The final product may still remain high in magnesium levels, and thus may not be used without difficulty by the fertilizer industry.
U.S. Pat. No. 3,554,694 to Barker et al teaches a process for producing commercially pure sodium fluosilicate from wet process phosphoric acid by reacting a sodium salt, e.g., sodium chloride, with fluosilicic acid present in the wet process phosphoric acid and precipitating sodium fluosilicates. The magnesium level may still remain high in the end product, so sludge problems may still occur when using the end product in neutralization processes.
U.S. Pat. No. 3,562,769 to Sugahara et al teaches purifying wet process acid by adding one disintegration preventing agent selected from the group consisting of alkali metal salts and sulfuric acid to calcium phosphate or phosphate rock; and heating the resulting mixture to 100.degree. to 300.degree. C. to convert the impurities to perfectly solid, non-disintegratable small masses; and thereafter extracting the phosphoric acid component from the masses. While the end product might be substantially clean, this process is complicated and may be too expensive to operate.
U.S. Pat. No. 3,935,298 to Sugahara et al teaches a process of mixing pulverized phosphate rock with fluorosilic acid, i.e., H.sub.2 SiF.sub.6, and then adding sulfuric acid to create a mixture which is then heat treated and shaped into non-disintegrating small masses. Further extraction of the small masses recovers phosphoric acid. H.sub.2 SiF.sub.6, it is taught, is a disintegration preventing agent which prevents the dried phosphate rock/sulfuric acid reaction product from disintegrating when phosphoric acid is being extracted therefrom.
U.S. Pat. No. 4,435,372 to Frazier et al describes a complex method of removing aluminum, magnesium and fluoride impurities from wet process phosphoric acid with the calcium sulfate hemihydrate filter cake by hydrolyzing and recycling the off gas scrubber solutions in the presence of a ferric ion catalyst. The patent teaches that controlling the quantitative ratios of the impurity components in the presence of the catalytic agent, ferric ion, will cause precipitation of undesirable impurity compounds in the acid. Potassium additions can be made. This process is complicated and fraught with many steps.
U.S. Pat. No. 4,136,199 to Mills describes a method of removing metal ion impurities, such as magnesium and aluminum, from phosphoric acid with an impure sludge, which contains calcium fluoride and which is obtained by treating waste pond water with lime or limestone. In one embodiment, concentrated wet process phosphoric acid having a P.sub.2 O.sub.5 content of 38 to 54 percent is mixed with a calcium fluoride-containing sludge and the resulting mixture is aged for five days following which it is centrifuged. The sludge introduces metallic ion impurities (col. 9, lines 1-3) and relatively high levels of aluminum or magnesium or both remain in the phosphoric acid product (Table 7). Overall the process is difficult to control because of varying compositions of the sludges used.
U.S. Pat. No. 4,299,804 to Parks et al teaches a process for precipitating impurities from unconcentrated acids, since, according to Parks et al, the high viscosity of concentrated wet process acid makes phase separation difficult and results in high capital expense for production equipment. The patent teaches that it is desirable to remove impurities in as large a quantity as possible, as early in the process as possible, without encountering processing problems. The patent teaches the addition to the filter grade wet process acid of a fluoride ion donating compound which may be hydrofluoric acid, sodium fluoride, sodium bifluoride, ammonium fluoride or ammonium bifluoride. H.sub.2 SiF.sub.6, Na.sub.2 SiF.sub.6 and Na.sub.3 AlF.sub.6 are stated to have been tested as possible sources of fluoride ion, but, according to the patent, did not yield the preferred effects. The main thrust of this patent is that precipitation of the magnesium and aluminum impurities should occur before the acid is concentrated and that such precipitation can be controlled by using a fluoride ion donor. Silica in the acid is taught as inhibiting or preventing the aluminum ion from precipitating out of solution. The patent further teaches that it may be necessary to add alum or other aluminum ion donating compounds in order to obtain desired minimum fluoride ion to aluminum ion ratio and effective precipitations.
A very effective method of minimizing post precipitation in wet process phosphoric acids is to "stabilize" the acids via a series of process steps. One such method is taught in Astley et al, U.S. Pat. No. 4,487,750. This patent is incorporated in its entirety herein. The present application is intended to improve on the Astley et al process in those instances when it is desirable that a stabilized acid also contain reduced levels of aluminum and magnesium. The basic steps of the stabilization process of this patent are summarized as follows:
(1) Concentrating unclarified, dilute, wet process phosphoric acid having a weight ratio of Fe.sub.2 O.sub.3 to P.sub.2 O.sub.5 weight ratio substantially higher than 0.03 in a first evaporation zone until the PA1 (2) Subjecting the concentrated acid solution to crystallization for at least 8 hours; PA1 (3) Clarifying the crystallized acid solution by centrifugation until its total solids content is less than 2%; PA1 (4) Concentrating said clarified acid solution in a second evaporation zone until the P.sub.2 O.sub.5 content of said clarified acid solution is between 58% and 63%.
P.sub.2 O.sub.5 content of the solution is between 45 and 55%;
The above procedure produces a stabilized acid that contains a maximum of 2% total solids any time during the first 28 days of either or both storage and shipment. There is no need to add aluminum silicate, oxidizing agents or any other additives, it is taught, to produce the desired results, and, in most cases, no additional equipment need be purchased. Magnesium contamination is still somewhat high in the end product and thus when neutralized with ammonia, magnesium salts precipitate out of solution.
U.S. Pat. No. 3,642,439 describes a method of removing magnesium from wet process phosphoric acid via formation of a precipitate comprising a magnesium-aluminum-fluoride-phosphate complex compound. The process requires that the SiO.sub.2 content be less than 0.2%, the F/MgO weight ratio be at least 2.2, and that the Al.sub.2 O.sub.3 /MgO weight ratio be at least 1.4. Aluminum and fluoride compounds must be added, if necessary, to maintain the ratios. A portion or all of the fluoride ion necessary to precipitate impurities may be obtained by decomposition, during evaporation, of H.sub.2 SiF.sub.6 usually present in the unconcentrated wet process acid to HF and SiF.sub.4 and the SiF.sub.4 is boiled off leaving HF present in the concentrated acid. The patent teaching recognizes that HF or a soluble fluoride additive needs to be added to the concentrated acid to raise the fluorine ion level.
The patent teaches against the addition of fluosilicic acid, H.sub.2 SiF.sub.6 to provide fluorine ions necessary for precipitate formation because, according to the patent, fluorine compounds, SiF.sub.4 or H.sub.2 SiF.sub.6, present in the acid are not effective to produce the desired magnesium complex. Addition of soluble fluorides capable of fluoride ion donation to the viscous concentrated acid, as taught by this patent, is not advantageous because the concentrated wet process phosphoric acid is supersaturated with impurities, is unstable, and therefore effective mixing of solubilized additives with the unstable acid requires extensive stirring and/or agitation of the precipitating solution. Moreover, if the concentrated acid is too concentrated, hydrogen fluoride will volatilize out of solution, it is taught.
Such a complicated precipitation step as is disclosed in the Moore et al patent is not necessary when sufficient fluoride may be added to the dilute solution at the beginning of the process as taught by the present invention. Moreover, the fluoride ion donating chemical does not have to be an expensive or caustic agent such as hydrofluoric acid. Furthermore, using seed crystals such as MgAlF.sub.5 in the precipitation step and aligning fluoride ions present in the dilute solution to the proportion of fluoride in the seed crystals can significantly reduce precipitation times and still effectively reduce the magnesium, aluminum, and fluorine to negligible levels. Accordingly, post precipitation and metal contamination may be more advantageously reduced by the present invention in the production of wet process phosphoric acid.
None of the prior art patents mentioned above disclose or suggest the addition of fluosilicic acid to a dilute wet process phosphoric acid followed by concentration, as by evaporation, of the dilute acid to a P.sub.2 O.sub.5 content of 45 to 55 wt. %, followed by crystallization of the magnesium and aluminum impurities, removal of the crystallized impurities, as by centrifugation, and final concentration, as by evaporation, of the acid to the desired P.sub.2 O.sub.5 content, e.g., at least 56 wt. %.
While prior stabilization processes may produce acids with low post-precipitation characteristics, the same stabilized acids may still have excessive quantities of magnesium and aluminum contained in them. Conversely, the aluminum and magnesium content of a wet process acid might be significantly reduced and the acid might still exhibit excessive post-precipitation characteristics. Thus, a need exists in the industry to produce a wet process phosphoric acid that is stabilized with respect to post-precipitation and which also has reduced levels of aluminum and magnesium contained therein.
It is thus an object of this invention to provide a process for producing wet process phosphoric acid with low post-precipitation characteristics and containing significantly reduced levels of magnesium and aluminum compounds.
Another object of this invention is to provide a process for stabilizing wet process phosphoric acid which process produces an excellent quality acid with respect to post-precipitation characteristics and low magnesium and aluminum contents from high-iron acid feeds as well as from low iron acid feeds without aluminum silicate addition and without oxidation or any other such special treatment.
Another object of this invention is to provide an inexpensive process for producing wet process phosphoric acid stabilized with respect to post-precipitation and low in magnesium and aluminum impurities.
Still another object of this invention is to provide a wet process phosphoric acid with higher-than-normal P.sub.2 O.sub.5 content in order that superior liquid fertilizers can be produced from the acid.
A still further object of this invention is to provide a process for stabilizing wet process phosphoric acid which process will not interfere with the simultaneous production of unstabilized normal merchant grade wet process phosphoric acid and unstabilized, merchant clarified wet process phosphoric acid.
These and other objects are accomplished by the process of our invention and will become apparent to those skilled in the art from the description that follows.